1. Field of the Invention
This invention relates to a spark plug.
2. Description of the Related Art
A spark plug used for ignition of an internal engine of such as automobiles generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A terminal metal fixture (terminal) is inserted into the projecting part of the insulator and is connected to the center electrode via a conductive glass seal layer which is formed by a glass sealing procedure or a resistor. A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode.
Under some combined conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharge called as a flashover occurs between the terminal metal fixture and the metal shell, going around the projecting insulator. Primarily for the purpose of avoiding flashover, most of commonly used spark plugs have a glaze layer on the surface of the insulator. The glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
In the case of the alumina insulator for the spark plug, such a glaze of lead silicate glass has conventionally been used where silicate glass is mixed with a relatively large amount of PbO to lower a softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing Pb have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out Pb glazes in a future, taking into consideration the adverse influences of waste spark plugs on the environment.
Leadless borosilicate glass- or alkaline borosilicate glass-based glazes have been studied as substitutes for the conventional Pb glazes, but they inevitably have inconveniences such as a high glass transition or an insufficient insulation resistance. To address this problem, JP-A-11-43351 proposes a leadless glaze composition having an adjusted Zn component to improve glass stability without increasing viscosity, and JP-A-11-106234 discloses a composition of leadless glaze for improving the insulation resistance by effects of joint addition of alkaline component.
Incidentally, since the glazes for spark plugs are used attaching to engines, they are apt to rise in temperature than cases of general insulating porcelains. Further, in recent years the voltage applied to spark plugs has been increasing together with advancing performance of engines. For these, the glaze for this use has been required to have insulation performance withstanding severer conditions of use. However, the glaze composition disclosed in JP-A-11-106234 is not always satisfactory in insulating performance at high temperatures, particularly the performance as evaluated as a glaze layer formed on an insulator in a spark plug (e.g., anti-flashover properties).
JP-A-11-106234 refers to the improvement of the insulation resistance by effects of joint addition of an alkaline component of the glaze containing Si or B as the glass skeleton component, but it could hardly recognized that a satisfactory attention is paid to a cancellation of differential thermal expansion coefficient in relation with the alumina based ceramics as composing ceramics of the insulator, and an improving level of the insulation resistance is not always satisfied.
It is a first object of the invention to provide such a spark plug having a glaze layer which has a reduced Pb content, is capable of being baked at relatively low temperatures, exhibits excellent insulation properties, and is easy to get a baked smooth surface.
It is a second object of the invention to provide such a spark plug where reduced is the differential thermal expansion coefficient in relation with the alumina based ceramics as composing the insulator by adjusting an alkaline metal component in the glaze, thereby to make less to cause defects as cracks or crazing in the glaze layer and farther heighten the insulation resistance.